Question

The factors of X^4-13x^2+36 when factored completely are

I did x^2(x^2-4)(x^2-9)
(X+3)(x-3)(x+2)(x-2)

it was multiple choice
1. (x^2+9)(x^2+4) 2. (x^2+9) (x+2)(x-2)
3. (X+3)(x-3)(x+2)(x-2) 4. (x+3)(x+3)(x+2)(x+2)

my answer is choice 3. Thanks for checking my work

Answer 1

What you did in
x^2(x^2-4)(x^2-9)
(X+3)(x-3)(x+2)(x-2) is incorrect.
what happened to the x^2 in front?

Here is the proper way:
x^4-13x^2+36
= (x^2 - 4)(x^2 - 9)
= (x+2)(x-2)(x+3)(x-3) , which is indeed #3

Even though you came up with the right answer, I would have given you only part marks if this had been my test question.